European Terrestrial Reference System 1989 (ETRS89)
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With each of the three GNSS TRFs we have encountered so far (US DoD tracking stations, broadcast GNSS orbits, and IERS/IGS TRF) a new version of the WGS84 datum has been introduced. Geodetic datums are like this – in theory the datum is exactly specified by the adopted conventions but in practice, each TRF intended to realise that specification actually implements a slightly different datum. Often, there are deliberate reasons for this, as in the case of the deliberate random element (Selective Availability) that was at one time introduced to the WGS84 datum in the broadcast satellite orbits.
Another type of deliberate variation to the WGS84 datum definition is found in realisations that are intended to serve a particular geographic region for mapping purposes. As we saw in listed above, the WGS84 position of any particular point on the Earth’s surface is changing continuously due to various effects, the most important of which is tectonic motion. So WGS84 itself is unsuitable for mapping – the ground keeps sliding across the surface of any WGS84 mapping grid!
However, it is still useful to have a mapping coordinate system that is compatible with GNSS. This is done by selecting a particular moment in time (in geodesy a moment in time is called an epoch, which is an unusual usage of that word), and stating the WGS84 coordinates of points in the region of interest at that epoch, regardless of the time of observation. Remember that the Cartesian axes and ellipsoid of WGS84 move steadily such that the motion is minimal with respect to the average of tectonic plate motions worldwide. Fixing the datum epoch has the effect of creating a new datum definition (that is, a new set of Cartesian axes and ellipsoid location and orientation) which initially coincides exactly with WGS84, but from then on remains stationary with respect to the particular piece of the Earth’s crust where the fixed points are, while moving steadily away from the WGS84 axes and ellipsoid.
This adoption of a particular WGS84 epoch to remove the effect of tectonic motion has been done in various places in the world – in fact, everywhere WGS84 has been adopted for mapping. Examples of WGS84-like datums which are gradually diverging from WGS84 are North American Datum 1983, New Zealand Geodetic Datum 2000, and the Geocentric Datum of Australia. There is also a European example, the European Terrestrial Reference System 1989 (ETRS89), which as the name suggests is a datum that coincided with WGS84 at the moment in time 1989.0, and has been slowly diverging ever since, moving with the Eurasian land mass. ETRS89 is ideal for a Europe-wide consistent mapping and data sets and it is mandatory for any data set complying with the EU INSPIRE directive.
For every realisation of ITRS (e.g. ITRF97, ITRF2000, ITRF2005,…) there is an equivalent TRF associated with ETRS89, called ETRFyyyy (or yy pre year 2000) where yyyy is the year of the “parent” ITRF (e.g. ETRF97, ETRF2000, ETRF2005 and so on). However, the epoch of all the ETRFs is 1989.0 so they are closely aligned with each other. What can be confusing is that often the ETRF will be quoted with an epoch relating to the parent ITRF from which it was derived. For example the current realisation of the OS Net coordinates (see ) are of course in ETRS89 (epoch 1989.0) but the “official” designation would be ETRF97 epoch 2009.756. That is - the parent ITRF is ITRF97 realised from observations centred on epoch 2009.756 (00:00:00, 04/10/2009). This information regarding the “parent” ITRF is useful if transforming between an ETRF and other ITRFs.
The reason for a new ETRF every time ITRF is updated is to take advantage of the improvements in the ITRF realisation and also to keep the ETRS89 realisation as close as possible to the current ITRS one, but still at epoch 1989.0.
Although not identical with WGS84, these locally-fixed GNSS datums are very easily and accurately related back to WGS84. This is because tectonic plate motion is very steady, predictable and precisely known. The ETRS89 coordinate of any point can easily be converted to a WGS84 coordinate via a simple transformation.
The importance of ETRS89 and ETRF to us in Britain is that this is the datum and TRF used for all OS GNSS positioning. It is a convenient system because we can forget about the tectonic effects apparent in WGS84 (which do not concern us in British mapping), while still being able easily to convert these coordinates to WGS84 when required. OS Net uses ETRS89 as its datum, and is a densification of the ETRF. More information on this topic is available in .